A single RADIALIS -like repressor controls floral heteranthery in Torenia

Floral morphology is precisely regulated to ensure successful reproduction. The formation of floral heteranthery, a key adaptive trait for pollination first recognized by Müller and Darwin over a century ago, remains poorly understood at the molecular level. Here, we studied the function of RADIALIS (RAD)-like genes in the bee-pollinated species, Torenia fournieri and its closely related weedy relative T. crustacea, uncovering their shared role as repressive modulators of floral heteranthery. Transcriptomic analysis of T. fournieri revealed a global suppression of gene expression in lateral stamens, where TfRAD1 is predominantly expressed. Loss-of-function tfrad1 mutants exhibit dramatically elongated lateral stamens, resulting from increased filament epidermal cell length and a genome-wide reactivation of genes normally repressed in wild-type lateral stamens. Conversely, ectopic expression of TfRAD1 is sufficient to shorten the ventral stamens, demonstrating that TfRAD1 is both necessary and sufficient for floral heteranthery. Furthermore, we developed a CRISPR/Cas9 system in T. crustacea, and generated the tcrad1ab mutants, confirming that TcRAD1 homologs play a conserved role in repressing lateral stamen elongation. To summarize, our study identifies a single RAD-like MYB protein as a co-repressor in stamen patterning.